I'd say an Anubis would be much better if upgrading him would make him immortal the turn he is played, rather than lowering the ability cost, because without quint he is just too vulnerable. You either have to play a number of anubises (to play multiple at once if opponent can eg. lobotomize, or gravity pull), or play a quinted FFQ/pharaoh to drain creature control spells before playing an anubis... first is not an option in other decks than time/aether, second requires you to play quint anyway...
And anyway you don't really have to immortalize more than a few creatures (oty, ffq), so anubises ability is not really overpowered... so I guess making him immortal the turn he is played (and increasing the cost to 10
) when he is upgraded would make him approximately as good as quint is. (quint - 3 quantum to immortalize one creature, anubis - 10+2x to immortalize x creatures) it seems balanced to me. There would be some cases when Anubis would be better than quint, but quint would still be enough in most cases and would still be better in rainbows due to low cost.